Method of producing pure single crystals of graphite



included within the graphite crystal structure.

' The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to a process for producing single crystals of graphite of high quality.

The process consists essentially of dissolving carbon under conditions of high pressures and high temperatures in a suitable-non-metallic solvent. Solution of carbon is carried out in a fused salt, or mixture of salts, to a state of saturation. The solution is thereupon cooled (while maintaining elevated pressure) to induce precipitation of free carbon, which recrystallizes as crystals of graphite. The temperature of the reaction is then reduced to normal, followed by lowering of pressure to normal. It has been found that pressures and temperatures in the order of several thousand atmospheres and hundreds of degrees centigrade, respectively are ,desirable in carrying out this process.

It is therefore an object of this invention to provide a method whereby carbonaceous materials may be converted to graphite crystals of high quality.

It is a further object of the invention to provide a method for producing graphite crystals by dissolving carbonaceous material in fused salts at high temperature and pressure and crystallizing the graphite from the solution. i .1 It has been found that the carbon dissolved at'high pressure andtemperature by the classes of solvents described inthis process is completely precipitated from solutionf In addition, the solvents are readily soluble in either water or dilute common acids. Since carbon is essentially inert chemically, the solvent salts may be readily separated from the recrystallized graphite end product, and recovered for later use.

Some solvents which have been found effective for the solution, precipitation and recrystallization of carbon as .welhformed graphite crystals at high pressures and high temperatures are ammonium halides, ammonium phosphates, ammonium sulfates and borate salts. Some re- United States Patent and 1000 to 2000" C., respectively, would be sufficient.

Analytical techniques used to evaluate reactions and reaction products consists of the various aspects of microscopy, spectroscopy and X-ray dilfraction.

Pressures and temperatures to be given below have been determined on the basis of electrical resistance methods on pure bismuth, thallium, barium and manganin alloy as a function of pressure. Temperatures have been measured by the use of platinum-platinum (13% rho dium) thermocouples and the pressure-corrected melting points of pure gold, nickel and platinum.

In carrying out the process of dissolving carbon and its recrystallization, the general procedure is to place a charge of solvent, for example, ammonium chloride, either pure or with various amounts of carbon or other salts admixed, within the bore of a polycrystalline carbon hollow right circular cylinder provided with carbon end plugs. However, any carbon capsule geometry which isolates the salt from other constituents in the high pressure cavity of the apparatus would be suitable. The saltcontaining carbon capsule is then placed withinthe bore of a section of compressible, fine grained, thermally and electrically insulating solid such as pyrophyllite alumina or magnesia. The latter assembly is then placed within the reaction cavity of the high pressurehigh temperature apparatus, as in the general arrangement disclosed in Patent No. 2,995,776, and pressure is applied and main tained on the specimen-for example, 60,000 atmospheres. The temperature is then raised, for example to 1600" C. either by passing electrical current directly through the carbon capsule or'through a surrounding furnace resistance heater coil or tube. Heat is applied to the specimen until the solvent fuses and dissolves carbon. After the desired degree of solution of carbon into the fused salt or salt mixture, the temperature is lowered in order to induce saturation and precipitation of graphite from the carbon-salt solution. During this part of the process the pressure is maintained on the capsule until it cools.

Other variations of pressure and temperature control to bring about carbon saturation and precipitation may be used; for example, maintaining a steady-state high temperature on the fused salt to achieve carbon supersaturation of the solution with subsequent decomposition and precipitation of free carbon for recrystallization to crystals of graphite. The former method, however, has been found to be preferable because of the reduced probability of contamination by diffusion of foreign elements present in the reaction cavity.

Upon completion of precipitation and recrystallization of carbon from the carbon-salt solution, the temperature ments which form stable carbides or carbon compounds I under conditions of high pressures and temperatures, are suitable as carbon solvents in this process.

' If crystals of chemically pure graphite are desired, it is recommended that chemically pure reactants are used, as well as carbon solvents which do not contain elements prone either to form carbon compounds or to be easily Ammonium chloride has been found to be especially effective as a carbon solvent for producing graphite crystals of similar to that described in US. Patent No. 2,995,776

(Giardini and Tydings); however, any device capable of generating and. maintaining concurrent pressures and temperatures in the order of several thousand atmospheres is lowered to normal, followed by reduction of pressure to normal. The sample is then recovered. The unreacted carbon is scraped away or otherwise removed and the salt-recrystallized graphite mixture treated with a solvent to wash away the salt (e.g warm water for NH Cl) and cleanse the graphite crystals.

Graphite crystals of high chemical purity and excellent crystal perfection have been grown by this process. Fundamental causes for the success of the process are .not fully understood. It is thought, however, that both the ionic nature of the solvent and the fluidity ofv the solution under the conditions of high pressure and high temperature used contributed to the high carbon solubility and complete precipitation and recrystallization to the dissolved carbon.

It has been found desirable in some cases to jacket the carbon-salt reactants with a thinwalled metallic container prior to installation within the high pressure-high temperature apparatus in order to prevent possible contamination of the reactants by intrusionof the surroundingcompressible thermal and electrical insulating solids which afrg'iu ss la r-11 p jackets has been found elTectivej to' pr -event injeetior jonflflow'age of environmental constituentsIwhen conditions recovemed.v

' the-sample recovered 'by establishedftechniques.

spat airs. f 11' 1 as of excessive shear or temperatures are encountered] within the-reaction cavity; Prudent-{selectionjofmetals' must laboratory. are given below I I I I V -mp1s .1

fully enclosed, by a containerof highpurity carhon, -for examplaf. spectrographic grade cgirhonaw ,The latter is f 1 pressurized ito-;approXimately 60,000 atmospheres, heatedto approximately 1650*" C, until fusion ofvthegsalt; and ji the desired degree. of carbonIsolation .l as' ;occu'r r'ed, (the V latter can be readilymonitored empirically by large 7 changesgiuthe electricalcharacterist cs of thefcircuitiwhen current is'passed dire'ctl-y through' the carbon of'thereac v tion,'or;hy thermocouple circuits otherwise), and then the reaction isquickly cooled to approximately 750C; so as to cause: precipitation and recrystalliiation of carbon from the solution in the' form of graphite crystals,@Theitemperature is} then lowered to normal followed bylowering of pre ssurieto iormalfgExtractiye andgreeove'ryjproj I cedureshave beendescribed earlier; 1, ,j .l v I C. only until tusionfof the salt p and sufficient s'olutio Qn h s .,i r LeX PlQ; h ,at' TO C f approximately one-halfminute) and then cooledtoroom' I temperature ,over' a period of approximately 5 minutes.

The pressureis then lowered to normaland thespecimen J mpires! A'charge of dehydrated sodium horate is'pla'ced withinI a pure carboh edntainehQ The-Ilatter' in turn, is placed 1 within -a thin walled' faiita'lum :tuhe. The specimen -placed -withir1;-the "reaction cavity and pressurizedetofapproximately 65 ,000 atmospheres and thereupon heated to about'1600 C. hel'dfo r one minute'and-cooled to room 7;

land

temperature. The pressure isth'en reducedto norfm' A "mixture" offiammonium iodide; and ammonium 1chlo; ride is. placed withinf 'a Iggraphite; ;capsu1e,;;. subjected, to, 70,000 atmospheres of pressure, heated tolapproximately,

' 1700.C. a'nd quenehedJ Pressur e isz redueedl to. normal and sample recovered.

Ktn'ixture of arnmomu f hexamirie and carbon :.are' placed within aQgra hite (container; subjected to about-75 .000 atmospheres of pressure, 7, heated uutil a'fsignificant ndabruptchangeinielectrical 1 characteristics of thelreactants occur, reflectingjfusion; I ofthe salts gnaw-mach; of car'l50n, thereupon {cooled toif room" temperature followed bya lowering ofjthe pressure to normaland'recovery of the j sample. I

" i Example- :6

ther sin j I p 'for chemical use 7 We claim;

drated I sodiumgboratey v enclose is ammoniumiodide.

isainmo .15.; Theip'rocess of claim l wherein; said; container is enclosed in aja ck et of 'metalffromthe group consisting o 'n 'at u I is appliedf graphite. j

mmonium iodide, ammonium Some usestor thelgraphite crystalsresultin'frorn the above process are: r I

I (41 For use in the construction of precision electrical resistance elements in electronics.

7 J (b) For use as "refractory liners in high temperature applicationssuch as rocket'engiries. f

Y .(c) 'For' use asj'control elements in nuclear r'eactors.

(d) As a source of graphite of highchemical p'urity (e) As a sourceof graphite,of hcontrolled chemical purity-and high structural perfection?forvstructural."and

fabrication uses.

Various modifications -of the pi'lo'cess 'steps" and/or the I ingredients maybe made without departingfrom the scope tofythe invention as definedin'the following claims;

7 1; flhe process of plreparinglgraphite erystals compris ing: charging a container of ca por with a solvent for Carlson from-theg'rpup consisti'n of ammonium chloride,

ammonium iodide, ammonium bromide, vammonium lithium chloride, ammonium 'di hyd'rogen phosphate, dehycobalt chloride dianiin'c,

tainer to approximately 160010.; cooling said container to; roorn emperature whilj'e maintaining said pressure;

32... The process] ofclaim f 1 :wherein saidi eontainer is charged witht a mixture o t saidcarbo n solvents from said.

. 'group,v Y

13 The process of whereinvsaidjcar bori solvent 1 s a mixture of ammonium iodide anclammo'nium chlo-v i f ide. I 1 1 4. Thejprocess'of iclaiml wherein said earhonsolvent is'a'mixture'of ammonium chloride and nickel-chloride *hex m n U' pplied I r 7.1 The pr'ocessyof clairn wherein saidcarbon'solvent is ammonium chloride.

, 1 8.,The'proces'sof claim .whereinsaid carbon solvent I is ammonium di-hydrogemphosphate.T I

' 9';-The process of claim- 1 wherein said carbon solvent isQdehydrated SQdium,borate.

10. The pro'cess' 'of claim wherein sai'clcarbon solvent 7 t is 'ammonium bi iluoride. I l

1 The process of clai wherein said carboii solvent 12. L'lhe process Jot-claim. 1: wherein saidicarbons'olvent i am n ambmmi v I V is' cobalt chloridef diamine. j].

. 131, .The priojce ss'of claim. i tmrgiasi'a d c rblcn solvent molybdenurri'and nt'alum before. pressure 1,; l 16. The sse -For; claim 1 wherein saidigcontainer is i References thel lhaininer I V v UNI D- STATES, PATENTS}. I

r I *3/51'j'Zridgmanjet II -23'. io9.,1

:B I SI ick' ela, chloride fhexami'ne and a I v subjecting said co1itainer to about 0,000-atmospheres ofpressure; he'ating ,saidcom I eprocessiof claim 1 wherein said carborl solvent 

1. THE PROCESS OF PREPARING GRAPHITE CRYSTALS COMPRISING: CHARGING A CONTAINER OF CARBON WITH A SOLVENT FOR CARBON FROM THE GROUP CONSISTING OF AMMONIUM CHLORIDE, AMMONIUM IODIDE, AMMONIUM BROMIDE, AMMONIUM LITHIUM CHLORIDE, AMMONIUM DI-HYDROGEN PHOSPHATE, DEHYDRATED SODIUM BORATE, NICKEL CHLORIDE HEXAMINE AND COBALT CHLORIDE DIAMINE; SUBJECTING SAID CONTAINER TO ABOUT 60,000 ATMOSPHERES OF PRESSURE; HEATING SAID CONTAINER TO APPROXIMTELY 1600*C., COOLING SAID CONTAINER TO ROOM TEMPERATURE WHILE MAINTAINING SAID PRESSURE; AND REDUCING SAID PRESSURE TO NORMAL 